1. Field of the Invention
The present invention relates to apparatus and method for diagnosing an exhaust gas recirculation system for an internal combustion engine, the exhaust gas recirculation system being disposed in the engine so as to recirculate part of exhaust gas to a suction system of the engine.
2. Description of the Background Art
An exhaust gas recirculation system which recirculates part of engine exhaust gas into an intake manifold so as to reduce a maximum combustion temperature in each combustion chamber of engine cylinders in order to reduce a harmful component of NOx (Nitrogen compound, x is for example 1, 1/2, 2/3 or so forth) in the exhaust gas is exemplified by a Japanese Patent Application First Publication No. Heisei 4-81557 (published on Mar. 16, 1992).
Due to a failure in the exhaust gas recirculation system, the exhaust gas recirculation is not carried out under an engine driving condition such that the exhaust gas recirculation (hereinafter, referred often to as EGR) should be carried out. On the contrary, due to the failure in the system, the exhaust gas recirculation is carried out under the engine driving condition such that the exhaust gas recirculation should not be carried out. Consequently, a sufficient reduction in an NOx exhaust gas quantity cannot be achieved and an engine driveability becomes worsened.
It is, therefore, desired that apparatus and method for diagnosing, with a high diagnosis accuracy, whether the exhaust gas recirculation system operates normally or malfunctions (operates abnormally) are developed.